A team of engineers that includes researchers from Sandia National Laboratories is working on a device to help ensure captured carbon dioxide stays deep underground.

The wireless device uses sensors to monitor for CO2 leaks and alert above-ground operators if a leak occurs. The device could operate for up to 40 years.

In carbon sequestration, CO2 is expected to be stored anywhere from 3,000 to 12,000 feet below the surface in an area that once contained oil, gas or water. A hole would be drilled through an impermeable layer of rock called cap rock that can prevent CO2 from percolating up toward the surface. Pressurized CO2 heated to around 175 degrees Fahrenheit would be pumped down this borehole. Once the storage area is full, the borehole would be plugged; in some cases, the trapped CO2 would react with the rock and bind permanently.

The researchers plan to embed glitter-sized CO2 sensors, about 1/8 of an inch by 1/8 of an inch, in the concrete surrounding the borehole, above and below the cap rock layer. Engineers from the California Institute of Technology are leading a group making the glitter-sized CO2 sensors. And chemists at Research Triangle Institute are leading a group making a coating to protect the sensors from the concrete, while still allowing CO2 to reach the sensors.

Sandia’s primary role is to make an electronic device that charges the CO2 sensors, receives information from them about the presence or absence of CO2 and sends that information up to operators at the surface. This smart collar device needs to work for anywhere from 20 to 40 years, researchers said.

Communication with the CO2 sensors works like the radio-frequency identification chip in a tap-to-pay credit card. The smart collar emits energy at one radio frequency to power the CO2 sensors. The sensors collect data on the amount of CO2 around them and send that information to the smart collar at a different radio frequency.

One of the biggest technical challenges the team had to overcome was the fact that these RFID chips aren’t designed to be embedded in concrete.

Radio challenges

In order to power the sensors through concrete, the team needs to focus very intense radio waves of a certain frequency at the sensors. However, much of these radio waves reflect off the concrete, drowning out any information from the sensors at that frequency. One workaround was to power the sensors with one frequency and then use far less intense radio waves of a different frequency to query the sensors and receive information back from them. This reportedly worked well in tests.

Recently, the Sandia team successfully showed the smart collar prototype powering and communicating with off-the-shelf RFID chips embedded in an inch of cement. For the smart collar to last for decades, the team designed the prototype to use supercapacitors to store power rather than batteries that only last for a couple of years. Next, the team will test the smart collar prototype with the Caltech-designed CO2 sensing chips.

The Sandia team has also tested powering and communicating with their smart collar prototype through 160 feet of commercially available wired pipe. This pipe had coaxial cable, similar to that used in cable TV, embedded within it, so that the system won’t need any other wires or cables that could introduce new escape routes for the CO2.

In 2023, the goal is to demonstrate the whole system first at Sandia’s above-ground testing facility and then at an underground test facility operated by the University of Texas at Austin. 

The project is funded by the Department of Energy and managed by the Office of Fossil Energy and Carbon Management and the National Energy Technology Laboratory.

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